The Colossi

What are the Colossi?

The most obvious question to ask about the Colossi is this - what exactly are they? Their appearance doesn't give any significant insight into their nature, they may be alive, or not, they may be magical, or not. However, when Wander defeats them, what they leave behind does give us some hints. Since it is in their death that we learn the most about them, in analysing them I will start at their death and work backwards.
When a Colossus is defeated they leave behind three basic components. The first, and most obvious, is the Dark Magic tendrils which move from the Colossus' body into Wander. This Dark Magic seems to emerge from within the Colossus, darkness encompassing its body while the tendrils go after Wander. Dormin states that it was severed into sixteen pieces and sealed away. There are sixteen Colossi, and each yields Dark Magic which enters Wander, and Dormin later emerges complete from Wander. Obviously these sixteen segments were sealed in the Colossi, meaning that they are the vessels within which Dormin was imprisoned by whoever sealed it away, and the Dark Magic we see each time a Colossus is defeated is a freed fragment of Dormin's power.
The next sign we see of the defeated Colossi is after Wander regains consciousness in the Shrine of Worship and leaves to find the next Colossus. In the distance there is a pillar of light in each area where a defeated Colossus lies. This is obviously no ordinary light, as it can be clearly seen during the day; it bears a resemblance to the beam cast by the Sword as it locates Wander's next opponent, suggesting that it is actually Light Magic. On closer examination we find that this pillar of light originates from the physical remains of the colossus. Since this Light Magic seems to be in some sense leaving the body of the colossus in much the same way that the Dark Magic did, Light Magic is evidently also a significant aspect of their nature.
As for the remains themselves, in terms of size and vague shape the Colossus is intact, but it is no longer a Colossus. What remains is a pile of earth and stone, somewhere between the Colossus' original form and a natural formation. Since the entire body of the Colossus is present, though altered in form, this appears to account for their physical makeup, where the Light and Dark Magic account for the metaphysical, making this the final component of the Colossi.

Given that the Colossus' body turns into earth when it is defeated and the magic leaves it, and that the remains appear to account for the entirety of its physical form, it is probable that this is actually what the Colossi are made of; that they are actually shaped from earth and stone, with the magic in their design giving them both life and their living appearance. This establishes that they are largely magical, that the Light and Dark Magic are responsible for most of what Wander fights against. What it does not answer is how each type of magic relates to the Colossus. This is more difficult to speculate intelligently on in isolation, so I will move on to the next aspect of the Colossi, which is their relationship with the being they were sealing away.

Connection to Dormin

In attempting to determine how exactly Dormin and the Colossi are tied together there are a number of observations and statements which we can draw from the story. Dormin claims to have been separated into sixteen pieces and sealed away in the distant past. Dormin's physical form is Dark Magic that emerges from within Wander, which originally entered him after leaving the sixteen defeated Colossi. As I mentioned above, this tells us that the Dark Magic in each Colossus is a fragment of Dormin's power, and these fragments are reunited in Wander to yield the freed Dormin.
This also tells us something about how the two types of magic relate to the Colossi. Dormin wanted to be free, he wanted the destruction of the Colossi, which is why he sent Wander to do so. However the Colossi fight back, despite the fact that some of Dormin's power is within them. This tells us that the Dark Magic in the Colossi does not have any control over them, which makes sense intuitively, since the Colossi were meant to seal that power away. Thus it must be the Light Magic which gives them their, for lack of a better term, 'personality'. Their minds, their awareness, is based off the Light Magic used to create them.
There is more to the Dark Magic aspect of the Colossi however. They die a 'magical death', the physical injuries Wander inflicts would be nothing to such large creatures, it must be some change caused by removing the sigils which kills them. If it was removing the Light Magic that did it, we would expect to see the pillar of light leaving their bodies straight away. Instead what we see is Dark Magic. When they are injured black blood gushes from the wound, when they are killed the immediate change is that the Dark Magic leaves their bodies. It is when this fragment of Dormin's power leaves the body that they die, suggesting that rather than simply being something they are sealing away, it is actually something they need to live.

Taking all this together, it seems that the Dark Magic is what is sustaining the Colossus' life, while the Light Magic gives them their form and their awareness. The Light Magic must also be what seals the Dark Magic within the Colossus - since the other two aspects are the Dark Magic itself, and the earth to which the Colossus returns. Breaking this seal allows the magic which sustains the Colossus to escape - and so damaging the seal deals it a significant injury, which is why they react so violently when the sigils are stabbed with the sword, despite being unperturbed by anything else.

The Seal

Having examined the main components of the Colossi and how they relate to each other, we can now move on to analysing their purpose, the seal which they embody. This seal has three main visible aspects to it; the sword, the sigils on the Colossi which Wander attacks, and the statues in the Shrine of Worship. Each one of these is also visibly associated with Light Magic; the sword uses a beam of Light to find its targets, the sigils glow in the presence of the sword, and the statues glow brightly in a similar fashion just as they are destroyed. It is thus obvious that the Seal is some form of Light Magic, used to contain and suppress Dormin's Dark Magic.

In order to kill the Colossi they must be attacked in certain areas with a certain weapon. These areas are areas where there is a Sigil, and the weapon that must be used is the sword which has some connection to Light Magic. Thus it seems that these sigils are the focal points of the seal on each Colossus and that, as the sword is capable of breaking the seal, attacking the sigils with the sword damages and eventually removes them, removing the seal.
When the Colossus is defeated, the Dark Magic only emerges from it at the points where the sigils were located. This suggests that these are the only points on the Colossus' body where the Dark Magic can leave. While the Colossus is already dead at this point, the Light Magic aspect must still be present, since it retains its original form; it is only later that the earthen body and Light Magic are seen separately. Since we know Light Magic was used to seal the Dark Magic, and it appears that the Dark Magic cannot physically penetrate the intact Colossus, it would appear that this Light Magic aspect physically prevents the movement of the Dark Magic while it is still present - so the only points where the Dark Magic can leave the Colossus are the points where this physical aspect of the seal was destroyed by the sword.
It can be seen that there is a more abstract aspect the Seal as well. While after the Colossus' defeat the Dark Magic is able to leave its body through the points where the sigils were located, on Colossi with more than one Sigil destroying just one does not free the Dark Magic. All must be destroyed before the Colossus is defeated and the Dark Magic is freed. This means that as well as the physical aspect of the Colossus body being impenetrable to the Dark Magic it contains, the Seal must prevent the Dark Magic from leaving the Colossus even through damaged areas, as long as the Seal itself is intact.
So, bringing all this together, there are two aspects to the Seal on the Dark Magic - a purely magical aspect which contains the Dark Magic within the Colossi apparently regardless of damage, and a physical aspect which is able to contain the Dark Magic anywhere it has not physically been damaged, even if the Seal is broken.

This covers how the Seal appears to function in magical terms. Another side which must be looked at is the more practical - the Colossi were, for the most part, designed to defend themselves. This must also have a place in the sealing away of Dormin, however the Colossi are not actually very effective at defending themselves. The main problem Wander faces when fighting them is actually getting to the Sigils, rather than avoiding the Colossus' attacks.
What needs to be kept in mind is that it is only the sword the allows Wander to defeat the Colossi so easily - to do so with normal weaponry would be very difficult, if possible at all, and the fact that the Dark Magic cannot escape until all the Sigils are removed shows that merely damaging the Colossi would not be enough.
Since it would be so difficult to defeat them due to their sheer size, they do not need to rely on fighting off opponents. Their attacks would be more to deter opponents, so that they at least do not present a stationary target. Some were designed with alternatives to this - one Colossus cannot attack at all, instead it depends on remaining in the air almost permanently, which serves as a similar deterrent to any attacker.